st: comparing trends

We have run an experiment where we have subjects search for a target item among differing number of distractor items (4, 7, 9). So we are interested in the trends in the amount of time it takes the participant to find the target as a function of the number of distractors (also called set size). Some times, the target is present and sometimes it is not. How do we do a comparison of the trends for target-present versus target-absent?
Scott Adler
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